1. Field of the Invention
The present invention relates to a pattern writing method for writing a pattern on a photomask or on a semiconductor substrate, for example. More particularly, it relates to a pattern writing method employing an electron beam writing device of variable-shaped vector scan system.
2. Description of the Background Art
FIGS. 12A, 12B and 12C are schematic views illustrating a pattern writing method in the background art employing an electron beam writing device of variable-shaped vector scan system. For writing a pattern F illustrated in FIG. 12A, a first shot is given to write a pattern in a rectangular region F1 (FIG. 12B) and thereafter, a second shot is given to write a pattern in a rectangular region F2 contiguous with the region F1 (FIG. 12C). The pattern F is thereby obtained as a combination of the two rectangular patterns. The irradiation time period with electron beam (hereinafter referred to as xe2x80x9cshot periodxe2x80x9d) for writing the pattern in the region F1 and the shot period for writing the pattern in the region F2 are the same.
When a plurality of shots are given to write a pattern, a deviation of the finished dimension of the pattern from the designed dimension thereof may occur. Especially when the pattern writing method involves the step of writing the pattern in the region F2 by giving a small shot size W in a Y direction as shown in FIG. 12C, the finished dimension of the pattern F is determined to be larger than the designed dimension thereof. This deviation supposedly results from different beam profiles of electron beams used for irradiating the regions F1 and F2, from variation in current density for each shot size, and the like. In FIGS. 12A through 12C, a vertical and horizontal directions in the plane of the drawing are defined as xe2x80x9cX directionxe2x80x9d and xe2x80x9cY directionxe2x80x9d, respectively.
A shot rank correction is known as a technique for compensating for deviation of the finished dimension caused by the shot size. More particularly, in the shot rank correction, the shot period (namely, the amount of dose) is varied according to the shot size and then pattern is written, thus compensating for the deviation. FIGS. 13A and 13B illustrate the shot rank correction technique. With reference to FIG. 13A, assuming that an isolated pattern is to be written, the dimension of an electron beam with respect to the Y direction for writing the isolated pattern is defined as a shot size S. In FIG. 13B, the horizontal axis of the graph shows the shot size S, and the vertical axis shows a deviation G. The deviation G is calculated by subtracting the designed dimension of the pattern from the finished dimension thereof with respect to the Y direction. It is seen from FIG. 13B that within the particular range where the shot size S has a small value, the finished dimension is smaller than the designed dimension. This result is caused by proximity effect. When a pattern is written with the shot size S2 and with the standard amount of dose bearing characteristic Q1, for example, a deviation xe2x88x92G1 is generated accordingly. This deviation xe2x88x92G1 is compensated for by giving the amount of dose bearing characteristic Q2 that is larger than the standard amount. The amount of dose to be corrected for each shot size S is obtained through experiment, for example.
According to the foregoing shot rank correction, when the finished dimension of the pattern is smaller than the designed dimension thereof, the amount of dose larger than the standard amount is given. As a result, a deviation of the finished dimension can be compensated for. In contrast, according to the pattern writing method in the background art illustrated in FIGS. 12A through 12C, the finished dimension of the pattern F is larger than the designed dimension thereof as described. Therefore, a deviation of the finished dimension of the pattern written by the background-art method cannot be compensated for by the shot rank correction.
When a pattern is to be written by giving a plurality of shots, it is an object of the resent invention to realize a pattern writing method suitably compensating for a deviation of the finished dimension of the pattern from the designed dimension thereof.
The pattern writing method of the present invention includes the following steps (a) and (b). In the step (a), a first region of an object to be written is irradiated with an electron beam for a first period, to write a first pattern having a fist side extending in a first direction, and a second side extending in a second direction. The step (b) is performed separately from the step (a). In the step (b), a second region of the object to be written is irradiated with an electron beam for a second period shorter than the first period, to write a second pattern having a first side overlapping the first side of the first pattern, and a second side extending in the second direction. The second side of the second pattern has a dimension of not more than a predetermined threshold value.
When the second pattern is written to have the second side of not more than the predetermined threshold value, the irradiation period with an electron beam is shortened. As a result, a deviation of the finished dimension of the pattern from the designed dimension thereof can be reduced.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.